To obtain hydrocarbons such as oil and gas, wellbores (also called the boreholes) are drilled by rotating a drill bit attached at the end of a drilling assembly generally called the “bottom hole assembly” or the “drilling assembly.” A large portion of the current drilling activity involves drilling highly deviated or substantially horizontal wellbores to increase the hydrocarbon production and/or to withdraw additional hydrocarbons from the earth's formations. The wellbore path of such wells is carefully planned before drilling such wellbores using seismic maps of the earth's subsurface and well data from previously drilled wellbores in the associated oil fields. Due to the very high cost of drilling such wellbores and the need precisely to place such wellbores in the reservoirs, it is essential continually to determine the position and direction of the drilling assembly and thus the drill bit during drilling of the wellbores. Such information is used, among other things, to monitor and adjust the drilling direction of the wellbores.
In drilling assemblies used until recently, the directional package commonly includes a set of accelerometers and a set of magnetometers, which respectively measure the earth's gravity and magnetic field. The drilling assembly is held stationary during the taking of the measurements from the accelerometers and the magnetometers. The toolface and the inclination angle are determined from the accelerometer measurements. The azimuth is then determined from the magnetometer measurements in conjunction with the tool face and inclination angle.
U.S. Pat. No. 5,091,644 to Minette having the same assignee as the present application teaches a method for analyzing data from a measurement-while-drilling (MWD) gamma ray density logging tool which compensates for rotations of the logging tool (along with the rest of the drillstring) during measurement periods. In accordance with the method disclosed therein, the received signal is broken down into a plurality of sections. In a preferred embodiment, the Minette invention calls for the breaking of the signal from the formation into four different sections: top, bottom, right, left. As the tool rotates, it passes through these four quadrants. Each time it passes a boundary, a counter is incremented, pointing to the next quadrant. This allows for dividing the data into four spectra for each detector. Each of these four spectra will be obtained for ¼th of the total acquisition time. The end result is an image of the wall of the borehole. In many situations, the data are recorded and stored downhole in a suitable memory and processed at the surface after the bottomhole assembly is tripped out of the borehole. In wireline applications, the data are telemetered to the surface and processed there with a surface processor or a remote processor.
There are many applications such as: real time geo-steering, borehole profiling, borehole stability, break out detection, dip and azimuth, fracture detection, etc., that require real time data and imaging. Bandwidth limitations in MWD are one of the most challenging problems for real time applications. Currently the bandwidth ranges from 4 to 20 bits/sec, with a bit error rate dependant on depth, mud weight, downhole pressure, pipe diameter, etc. The demand for real time data is growing much faster than the demand for bandwidth. Data compression and image compression provide a solution to maximize utilization of BW.
In order to improve reliability of the compression technique and thereby minimize transmission time, data type, drilling processes, and bit error rates have to be considered when designing compression algorithms. Formation data in LWD is continuously collected during the drilling process. The borehole shape can be described as a circle or as an ellipse. Image data is usually presented as a rectangular matrix (depth vs. azimuth). The horizontal axis represents azimuthal information while the vertical axis represents depth. Every depth increment represents a period of time where the tool has rotated one or more times. High accuracy data are only required for certain depth intervals in the well. Predetermined knowledge about the well is available from models, seismic data, or offset wells, which helps to determine those regions of interest.
MWD provides continuous data while drilling. The data needs to be divided into segments in such a way that segmentation does not occur at the middle of a feature (i.e.: bed boundary). Equally spaced depth segmentation may destroy the feature if segmentation occurs in the middle of a feature.
MWD provides continuous data while drilling. The data needs to be divided into segments in such a way that segmentation does not occur at the middle of a feature (i.e.: bed boundary). Equally spaced depth segmentation may destroy the feature if segmentation occurs in the middle of a feature.
MWD sensors are 20 to 100 feet behind the bit. Latency is defined as the time from when the sensors see the formation to the time when that information is available at surface. For the same signal to noise ratio, reducing the size of a data block reduces the compression ratio while increasing the latency.
Getting data from tool memory can take up too several days, (i.e.: pulling the tool to surface). If the tool is lost down hole, the data is lost.
There is a need for a method of real time imaging of data acquired downhole and extracting features descriptive of the data so that the extracted features can be transmitted uphole substantially in real time within the limitations of the telemetry system that is available. The present invention satisfies this need.